US4644238A - Airflow control apparatus - Google Patents

Abstract

An apparatus for controlling an airflow to a boiler or the like and maintaining a desired quantity thereof. The apparatus comprises a controller for generating a control signal of a level corresponding to a required airflow, a variable-frequency power supply for producing an output of a frequency corresponding to the control signal level so as to drive a ventilation-actuating motor, and means for detecting any deviation of the control signal level from a predetermined range and retaining the frequency of the motor-driving output at a proper value, whereby the airflow is maintained at or above a preset value.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to airflow control for a boiler or the like, and more particularly to an airflow control apparatus for use in a system where a ventilator is driven by a motor energized from a variable frequency power supply.

2. Description of the Prior Art

A conventional control apparatus of this type known heretofore has a constitution of FIG. 1, wherein there are shown acommercial power line 1,breakers 2 and 3, amotor 4, aventilator 5 connected directly to the motor, a VF (variable frequency)power supply 6, acontroller 8 for feeding an operating frequency signal to the VFpower supply 6, and asignal line 7 of thecontroller 8. FIG. 2 graphically shows the relationship between an operating frequency and a frequency command signal outputted from thecontroller 8.

FIGS. 3 and 4 are explanatory diagrams relative to FIG. 1, wherein: t₁ is a time when an increment command is outputted from thecontroller 8; t₂ is a time when increasing the output of theVF power supply 6 is completed; t₃ is a time when a fault occurs on thesignal line 7; and t₄ is a time when a change in the output of theVF power supply 6 is completed posterior to occurrence of the fault.

In such conventional control apparatus, the operation is performed in the following manner.

In FIG. 1, a voltage is applied from thecommercial power line 1 via thebreaker 2 to theVF power supply 6. The rotational speed n of themotor 4 is dependent on the output frequency F of theVF power supply 6 as given by Eq. (1) below. ##EQU1##

Thus, the rotational speed n varies in proportion to the frequency F of the applied voltage. It follows that a change in the output frequency F of theVF power supply 6 causes a change in the rotational speed of themotor 4. Theventilator 5 connected directly to themotor 4 sends to a boiler an output airflow Q which is substantially proportional to the rotational speed n of themotor 4. In an exemplary case of a power plant, when there arises a change in the demand from a power system or in a fuel feed condition, the airflow Q required for the boiler comes to vary as well, and a command signal for changing the airflow Q is outputted from thecontroller 8 via thesignal line 7 to theVF power supply 6, which then produces an output of a frequency F corresponding to the required airflow Q.

FIG. 2 graphically shows the relationship between the operating frequency and the frequency command current outputted from thecontroller 8. It is plotted in this gragh that thecontroller 8 produces a frequency command signal of a current from 4 to 20 mA for varying the operating frequency of theVF power supply 6 in a range of 10 to 40 Hz.

FIG. 3 is an explanatory diagram relative to a normal operation performed in the conventional apparatus of FIG. 1. Thecontroller 8 requests increase of the airflow Q at time t₁ and feeds a frequency command signal via thesignal line 7 to theVF power supply 6. Generally theVF power supply 6 is so designed as to increase or decrease the output thereof at a predetermined rate in response to the request for variation. Accordingly, the output frequency F of theVF power supply 6 is somewhat delayed with respect to the variation request time t₁, and the action to execute the variation request is terminated at time t₂. And the rotational speed n of themotor 4, hence the output airflow Q of theventilator 5, comes to vary in accordance with the output frequency F of theVF power supply 6.

FIG. 4 shows an example with a fault or abnormal state caused on thesignal line 7 of FIG. 1. In this case where the fault is disconnection of thesignal line 7, thecontroller 8 keeps requesting a predetermined airflow Q with its output remaining unchanged, but the signal line is interrupted at time t₃ so that no further input is fed to theVF power supply 6. As described previously in relation to FIG. 3, the output frequency F of theVF power supply 6 is decreased to its lower limit with some delay, and simultaneously the rotational speed n of themotor 4 is also decreased to bring about a reduction in the output airflow Q of theventilator 6.

In the conventional apparatus having the constitution described above, when the frequency command signal from thecontroller 8 becomes abnormal or comes to be extinct due to disconnection or other fault of thesignal line 7, theVF power supply 6 drives themotor 4 by the aforesaid fault signal. Consequently, some disadvantages have been unavoidable heretofore including that theventilator 5 connected directly to themotor 4 fails to maintain the required airflow, thereby inducing the possibility of extinguishing the fire in the boiler or even causing its explosion.

SUMMARY OF THE INVENTION

The object of the present invention resides in providing an improved airflow control apparatus capable of minimizing a variation induced in the rotational speed of a motor for a ventilator when a normal signal fails to be fed to a VF power supply due to generation of a fault signal from a controller or due to occurrence of disconnection of a signal line via which the output signal of the controller is fed to the VF power supply.

In brief, the airflow control apparatus according to the invention comprises a control circuit for generating a control signal corresponding to a desired airflow, a signal line for introducing the control signal to a control input terminal of a VF power supply, and a fault signal detector which checks whether the control signal on the signal line is normal or abnormal and, in the latter case, produces a command for supplying a drive current of a preset frequency to a ventilator-actuating motor.

In an exemplary embodiment of the invention, the fault signal detector has means to produce an output signal upon detection of any abnormality in the control signal, and means to switch the motor from the VF power supply to the commercial power line in response to the detection signal.

In another embodiment of the invention, the fault signal detector includes means to produce an output signal upon detection of any abnormality in the control signal, means to raise, in response to the detection signal, the output frequency of the VF power supply at a fixed rate to the vicinity of the upper limit of a predetermined frequency variation range, and means to switch the motor from the VF power supply to the commercial power line after completion of such rise of the frequency.

In further embodiment of the invention, the fault signal detector includes means to produce an output signal upon detection of any abnormality in the control signal, and means to lock, in response to the detection signal, the output frequency of the VF power supply at the value immediately anterior to detection of the abnormality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the constitution of a conventional airflow control apparatus;

FIG. 2 graphically shows the relationship between the input control signal current and the output signal frequency of a VF power supply in the apparatus of FIG. 1;

FIGS. 3 and 4 respectively show signal level variations induced in the outputs of individual components due to sharp rise and fall of the signal level via a signal line to the VF power supply in the apparatus of FIG. 1;

FIG. 5 is a block diagram of an exemplary airflow control apparatus embodying the present invention;

FIG. 6 graphically shows changes caused in the operating states of individual components when the control signal level sharply falls in the apparatus of FIG. 5;

FIG. 7 is a block diagram of another exemplary airflow control apparatus embodying the invention; and

FIG. 8 graphically shows changes caused in the operating states of individual components when the control signal level sharply falls in the apparatus of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an exemplary airflow control apparatus of FIG. 5 embodying the present invention, there are shown acommercial power line 1,breakers 2 and 3, amotor 4, aventilator 5, a VF (variable frequency)power supply 6, asignal line 7 and acontroller 8, all of which are equivalent to those shown previously in FIG. 1. Therefore, repeated description of such components is omitted here. In addition thereto, abreaker 9 is inserted in aline 10 which connects themotor 4 to thecommercial power line 1. In order to supply required electric power from thecommercial power line 1 to themotor 4, there are provided amain line 11 including theaforesaid breaker 2, VFpower supply 6 andbreaker 3, and also anauxiliary line 10 including thebreaker 9.

Afault signal detector 12 keeps monitoring the control signal level on thesignal line 7 and, upon deviation of the signal level from a predetermined range, judges the control signal to be abnormal and executes an action of opening thebreakers 2 and 3 while closing thebreaker 9. In an exemplary case where a control signal of a current level ranging from 4 to 20 mA as shown in FIG. 2 is fed from thecontroller 8 via thesignal line 7 to theVF power supply 6, if the current level varies to be below 4 mA or above 20 mA, thefault signal detector 12 judges the control signal to be abnormal and executes the aforesaid action. The same action is executed also in the case of disconnection of thesignal line 7.

It is possible for those skilled in the art to constitute with facility thefault signal detector 12 that carries out such action by a combination of a current-voltage converter, two voltage comparators and a switching means actuated by the outputs of the comparators.

Thefault signal detector 12 may further include a level control means which, upon detection of any abnormality relative to thesignal line 7 or the control signal being fed to theVF power supply 6, raises the level of the control signal at a fixed rate to a predetermined upper limit thereof regardless of the output of thecontroller 8 prior to opening thebreakers 2 and 3 while closing thebreaker 9. FIG. 6 shows the transitions of operating states of the individual components in such arrangement.

In FIG. 6: t₅ is a time when the output frequency F of theVF power supply 6 reaches the vicinity of the upper limit of a predetermined frequency control range; t₆ is a time when thebreakers 2 and 3 are completely opened t₇ is a time when the closing ofbreaker 9 is completed; t₈ is a time when the rotational speed n of themotor 4 reaches a value corresponding to the frequency of thecommercial power line 1; and t₉ is a time when the output airflow Q of theventilator 5 is stabilized at a steady-state value.

If disconnection occurs at time t₃ in thesignal line 7 between thecontroller 8 and theVF power supply 6, thefault signal detector 12 immediately detects such abnormality, then raises the output frequency of theVF power supply 6 to the upper limit thereof at a fixed rate, and subsequently opens thebreakers 2 and 3 (at time t₆) and closes the breaker 9 (at time t₇). The rotational speed n of themotor 4 increases gradually until the output frequency F reaches its upper limit (at time t₅), and then decreases due to opening of thebreakers 2 and 3 which temporarily interrupts application of the voltage to themotor 4. And when thebreaker 9 is closed, the rotational speed n increases again quickly to reach the value corresponding to the frequency of the commercial power line at time t₈.

For controlling the airflow Q of the ventilator, a mechanical inlet valve or damper (not shown) may be utilized as well as the aforementioned rotational speed n. In order to minimize the loss under control with the rotational speed n, the inlet valve is locked substantially at its fully open position. And when thefault signal detector 12 is actuated, the inlet valve is automatically opened or closed by an airflow control signal. That is, for the purpose of attaining a constant airflow, the inlet valve is narrowed down in accordance with acceleration of the rotational speed n so as to suppress increase of the airflow Q that may otherwise be induced due to the accelerated rotational speed n. Since the output frequency F rises gradually during a period between t₃ and t₅, narrowing down the inlet valve for the airflow Q is effected sufficiently in time, thereby maintaining the airflow Q constant during this period.

In FIG. 6, the output airflow Q once decreases after time t₆ with thebreakers 2 and 3 opened and then commences increasing at time t₇ with thebreaker 9 closed. The response of the inlet valve is usually delayed from increase of the airflow Q induced by fast acceleration of the rotational speed n. Accordingly, during such fast acceleration of the rotational speed n, the airflow Q increases somewhat slowly through the action of the inlet valve. And after arrival of the rotational speed n at a value corresponding to the frequency of the commercial power line (posterior to time t₈), the airflow Q is turned to decrease by the action of the inlet valve and thereby resumes the former value at time t₉. The output airflow Q is controlled in the manner mentioned above, and its variation is kept within a permissible range so that the operation can be continuously advanced in safety despite occurrence of any abnormality in the output signal of thecontroller 8 or on thesignal line 7.

Although the airflow control has been described above with regard to the ventilator for a boiler, it is to be understood that application of the present invention is not limited to the boiler alone. Moreover, the same control action can be executed in the case of any other fault or abnormal state such as short-circuiting than the aforesaid disconnection of thesignal line 7 taken as an exemplary fault. And in addition to such abnormality of thesignal line 7, any abnormal output signal that results from a fault in thecontroller 8 is also detectable for control.

Furthermore, besides the foregoing arrangement where the output frequency F of theVF power supply 6 is raised immediately after actuation of thefault signal detector 12, it may be so modified as to raise the output frequency F of theVF power supply 6 after maintaining the same unchanged for a predetermined period of time.

FIG. 7 shows a further exemplary airflow control apparatus embodying the present invention, wherein, in addition to the components employed in the foregoing apparatus of FIG. 5, thefault signal detector 12 has switch means 13 inserted in thesignal line 7 and turned off in response to detection of any abnormality in the signal being fed through thesignal line 7, and an automatic follow-up circuit 14 which follows with some delay a change in the control signal on thesignal line 7, then holds the control signal level immediately anterior to the actuation of the switch means 13 and, after turn-off of the switch means 13, continuously feeds the signal of the level being held to the control input terminal of theVF power supply 6.

The apparatus of FIG. 7 performs its operation in the following manner. When thefault signal detector 12 detects that the output signal of thecontroller 8 is beyond a current range of 4 to 20 mA graphed in FIG. 2 or no signal is existent on thesignal line 7 due to disconnection thereof, thesignal line 7 is open-circuited by the switch means 13. Then the automatic follow-up circuit 14 follows the output signal of thecontroller 8 with some delay and, upon interruption of the output signal, holds the signal level immediately anterior to such interruption. That is, when thefault signal detector 12 open-circuits thesignal line 7 upon occurrence of disconnection or the like of thesignal line 7, the automatic follow-up circuit 14 produces a signal to control the output of theVF power supply 6 at a fixed value. Consequently, the rotational speed n of themotor 4 and the airflow Q of theventilator 5 are maintained constant by the fixed output of theVF power supply 6.

FIG. 8 is an explanatory diagram relative to the above control action in an exemplary case where the output signal of thecontroller 8 has become nonexistent at time t₃ as a result of a fault such as disconnection of thesignal line 7. In this case, the abnormal state is detected at time t₃ by thefault signal detector 12, and the output frequency F of theVF power supply 6 is maintained constant by the fixed output signal of the automatic follow-up circuit 14. Consequently, despite occurrence of any abnormality on thesignal line 7, the output frequency F of theVF power supply 6 remains unchanged to maintain the normal operation at the signal level anterior to the fault of thesignal line 7, thereby retaining the rotational speed n of themotor 4 and the airflow Q of theventilator 5 without any change to eventually continue the operation in safety.

The airflow Q of theventilator 5 is controllable by the use of, besides the aforesaid rotational speed n, an inlet valve or damper (not shown) which is a mechanical airflow regulator. In such a case, the inlet valve is locked substantially at its fully open position so as to minimize the loss under control with the rotational speed n. And when a steady operation is attained with the rotational speed n rendered constant in the aforementioned manner after detection of an abnormality by thefault signal detector 9, the inlet valve is released from the controlled lock in response to a change in the airflow required for the boiler, and the control mode is switched for following up the required airflow, thereby keeping the boiler operation safe.

As is manifest from the detailed description given hereinabove, according to the airflow control apparatus of the present invention, an automatic control action is executed to hold the rotational speed of the ventilator-actuating motor at or above a predetermined minimal value upon occurrence of any fault or abnormal state relative to the control signal being fed to the VF power supply. Therefore, the present invention is applicable advantageously to any system such as a boiler where ventilation is requisite.

Claims (7)

What is claimed is:

1. In an airflow control apparatus having a ventilator-actuating motor driven at a rotational speed corresponding to a power supply frequency, a VF (variable frequency) power supply connected to a commercial power line for supplying an electric power to said motor, and a controller for feeding a control signal to said VF power supply so as to determine the output frequency thereof, the improvement comprising a fault signal detector including: means for detecting an abnormality relative to the control signal fed to said VF power supply and producing an output; and means actuated in response to the output of said detecting means and serving to actuate switch means for switching the input terminal of said motor from said VF power supply to said commercial power line; and level control means which is actuated in response to the output of said detecting means, said level control means raising the level of the control signal fed to said VF power supply to the upper limit of a predetermined variation range and subsequently actuating said switch means to connect said motor to said commercial power line.

2. The apparatus as defined in claim 1 wherein said control signal, upon detection of an abnormality, has a current level varied within a predetermined range in accordance with a required air flow.

3. The apparatus as defined in claim 1 and further including means for disconnecting said motor from said VF power supply before connecting said motor to said commercial power line.

4. The apparatus as defined in claim 2, wherein the current level of said control signal is varied within a range of 4 to 20 mA.

5. In an airflow control apparatus having a ventilator-actuating motor driven at a rotational speed corresponding to a power supply frequency, a VF (variable frequency) power supply connected to a commercial power line for supplying an electric power to said motor, and a controller for feeding a control signal to said VF power supply so as to determine the output frequency thereof, the improvement comprising: a fault signal detector for detecting an abnormality relative to the control signal fed to said VF power supply and actuating switch means to interrupt said control signal being fed to said VF power supply; and an automatic follow-up circuit for holding the level of said control signal immediately anterior to the action of said switch means and feeding said control signal of the held level to said VF power supply in succession to the action of said switch means.

6. The apparatus as defined in claim 5, wherein said control signal has a current level varied within a predetermined range in accordance with a required airflow.

7. The apparatus as defined in claim 6, wherein the current level of said control signal is varied within a range of 4 to 20 mA.

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